Various authentication technologies such as biometric authentication or gesture authentication have been known. In the biometric authentication, for example, there are cases where the authentication rate declines because the similarity between two images declines when the intensities of external light differs between capturing a registered image and capturing a matching image. Disclosed as an example of a technology for reducing the influence of external light is an image processing method for acquiring a difference between an OFF image captured during an OFF state in which the illumination is turned OFF and an ON image captured during an ON state in which the illumination is turned ON. The image processing method aims to acquire an image that does not include external light but illumination light by computing the difference between an OFF image captured with only the external light and an ON image captured with the external light and illumination light.
However, when the external light is strong, the pixel values for the portion illuminated with the external light within the area captured by the camera saturate at the upper boundary, and a phenomenon called saturation is thus produced in which such a portion becomes completely white. Therefore, when the difference between the OFF image and the ON image is simply acquired, the authentication ratio may remain low because the characteristics of the region to be biometrically authenticated are lost due to the saturation.
As an example of a technology for solving saturation, Japanese Laid-open Patent Publication No. 2008-90412, for example, discloses an image generating method for dividing an image capturing process into sub-steps based on the intensities of external light, alternately capturing OFF images and ON images, and integrating differences between respective pairs of the OFF image and ON image. This image generating method aims to acquire an image in which the influence of saturation is reduced by reducing the length of the exposure time during capturing of the images to reduce the amount of incident light and by integrating the captured difference images.
However, such a technology has a disadvantage that the time for capturing images is extended as explained below.
To explain, in the image generation method mentioned above, OFF images and ON images are captured alternatingly. When OFF images and ON images are captured alternatingly, the illumination is switched every time an OFF image or an ON image is to be captured. In other words, the illumination is switched from ON to OFF every time an OFF image is to be captured, and the illumination is switched from OFF to ON every time an ON image is to be captured. Because images captured during the process of switching the illumination are captured without the illumination not completely turned OFF or completely turned ON, and such images could be a cause for reducing authentication accuracy, it is preferable not to use the images in the authentication. Because capturing operation is withheld until the illumination is switched OFF or ON every time an OFF image or an ON image is to be captured, the image capturing time is extended.